Advisory [CVE-2026-68179] only unlock nsm_dev on post-lock error paths

In the Linux kernel, the following vulnerability has been resolved: misc: nsm: only unlock nsm_dev on post-lock error paths nsm_dev_ioctl() jumps to the common out label even when the initial copy_from_user() fails before nsm->lock has been taken. The error path then blindly unlocks a mutex that was never acquired.

This issue was found by our static analysis tool and then manually reviewed against the current tree. The grounded PoC kept the miscdevice ioctl entry and the pre-lock copy_from_user(&raw, argp, _IOC_SIZE(cmd)) failure path by issuing NSM_IOCTL_RAW with an invalid user pointer.

Lockdep reported: WARNING: bad unlock balance detected! exploit/193 is trying to release lock (&global_nsm.lock) at: nsm_dev_ioctl+0x5f/0xcf [vuln_msv] but there are no more locks to release! no locks held by exploit/193. Return immediately on the pre-lock copy_from_user() failure and keep the common unlock label for the post-lock paths only.

A flaw was found in the Linux kernel's Network Shared Memory (NSM) module. A local attacker can exploit an issue in the `nsm_dev_ioctl()` function where an incorrect mutex unlock operation is performed.

This occurs when an initial data copy fails, leading to an attempt to unlock a mutex that was never acquired. Successful exploitation can result in a kernel panic, causing a denial of service (DoS).

Red Hat severity: not rated. Weakness: CWE-832.